The invention relates to the field of transmitter and/or receiver antennas, optionally of the array type, and more particularly to orthomode transducer devices (or “transducers”) which equip such antennas.
“Antenna” is here understood to mean both a single elementary radiation source coupled to an orthomode transducer device and an array antenna.
Furthermore, an “array antenna” is here understood to mean an antenna that is able to function in transmission and/or in reception and comprising an array of elementary radiation sources and control means suitable for controlling, by means of (an) active system(s), the amplitude and/or the phase of the radiofrequency signals to be transmitted (or in the reverse direction, received from space in the form of waves) by the elementary radiation sources according to a chosen diagram. Consequently, it can equally be a so-called direct radiation antenna (often designated by the English acronym DRA), one that is active or more rarely passive, or active or passive sources of the array type located in front of a reflector(s) system.
Moreover, “orthomode transducer” is here understood to mean what the person skilled in the art would know by the acronym OMT, that is to say a device designed to be connected to an elementary radiation source, such as a horn, so as selectively to feed it (in transmission) or be fed (in reception) either with a first electromagnetic mode having a first polarization or with a second electromagnetic mode having a second polarization orthogonal to the first. The first and second polarizations are generally linear (horizontal (H) and vertical (V)). However, circular polarization can also be produced by adding additional components with a view to creating the appropriate phase states.
Such a transducer comprises for example:                a main (wave)guide designed for the propagation along a main (radioelectric) axis of first and second electromagnetic modes having first and second polarizations orthogonal to each other and provided with a first end (coupled to a circular port suited to the first and second modes and designed to be connected to an elementary radiation source) and a second end;        a first auxiliary (wave)guide designed for the propagation of the first electromagnetic mode along a first auxiliary (radioelectric) axis. The first radioelectric axis is collinear with the radioelectric axis of the main guide but is not necessarily coincident with it. The first auxiliary guide is provided with a first end, coupled in series to the second end of the main guide via a series window, and with a second end coupled to a series port suited to the first mode; and        at least one second auxiliary guide designed for the propagation of the second electromagnetic mode along a second auxiliary (radioelectric) axis, coupled to the main guide via at least one parallel window and provided with a first end coupled to a parallel port suited to the second mode.        
As the person skilled in the art knows, in an array antenna the space available for inserting radiating elements (or elementary radiation sources) depends directly on the size of the mesh (or the basic pattern) of the array, which is fixed by the operational needs (frequency band intended, performance optimization, reduction of losses by lobes of the array (in the case of a DRA), sampling of the focal spot (in the case of a reflector antenna and an array-type source)).
In the bipolarization applications intended here, and in particular when the bipolarization is linear, it is necessary to locate the orthomode transducer (OMT) just behind the corresponding elementary radiation source. Yet when the OMTs are produced with waveguide technology, their size in the plane of the mesh (perpendicular to the main axis) quickly becomes greater than that of the mesh (typically greater than or equal to 1.2λ, where λ is the operating wavelength in a vacuum). Specifically, in the most commonly used OMTs at least one second auxiliary guide is connected to the main guide (or body of the OMT) by a bend, although their size in the plane of the mesh is typically around 3λ. In this case there is incompatibility between the size of the OMTs and that of the mesh.
In the document by W. Steffe “A novel compact OMJ for Ku band intelsat applications”, IEEE Antennas and Propagation Society International Symposium, June 1995, AP-S. Digest, volume 1, it has been proposed to produce orthomode junctions (or OMJs) of reduced compactness. This type of OMJ comprises a main (wave)guide, of the aforementioned type, of square cross section and designed to be coupled via a series window to a first auxiliary guide in series (suited to the propagation of the first electromagnetic mode), and a second auxiliary guide of rectangular cross section suited to the propagation of the second electromagnetic mode, coupled to the main guide via a parallel window and provided with a first end designed to be coupled to a parallel port suited to the second mode. The parallel window is defined between a lateral wall of the main guide and a lateral wall of the second auxiliary guide (which extends over a height equal to that of the shorter side of its rectangular cross section), while the second auxiliary guide extends in the plane of the mesh over a distance equal to that of the longer side of its rectangular cross section. The OMJ therefore has a space requirement in the plane of the mesh typically of around 2λ, which still proves to be too high. In addition, the positioning of the ports then makes the architecture of the complete antenna much more complicated and has the effect of increasing the assessments of mass and size requirement.
No known solution is completely satisfactory; the invention therefore aims to improve the situation.